lib/lz4: make arrays static const, reduces object code size

Don't populate the read-only arrays dec32table and dec64table on the
stack, instead make them both static const.  Makes the object code
smaller by over 10K bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  31500	      0	      0	  31500	   7b0c	lib/lz4/lz4_decompress.o

After:
   text	   data	    bss	    dec	    hex	filename
  20237	    176	      0	  20413	   4fbd	lib/lz4/lz4_decompress.o

(gcc version 7.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>